14 ANNALS NEW YORK ACADEMY OF SCIENCES 



that is in the vicinity of Giianica. Limestones belonging to the older 

 series occur immediately south of Yauco and are very strongly developed 

 there. The hills in which these beds outcrop extend southward almost 

 continuoush' to Guanica, but at that point observations showed that the 

 formation had changed and is actually part of the reef limestone of the 

 x^recibo formation, although it is possible that the large fault observed 

 west of Ponce may pass through this area and obscure the other structural 

 relations. It looks, from the rapid survey, as though it would be favor- 

 able for some of these additional studies of the character of the lower 

 beds of the Arecibo formation. Between Guanica and Juana Diaz, wher- 

 ever the inner margin of the xlrecibo formation was seen, it was bounded 

 by a fault which brings the upper beds abruptly against the older tuffs 

 and shales of the pre-Tertiary. 



The formation furnishes an abundance of fossils. The lower portion 

 or the lower* beds on the south side of the island, as seen at Juana Diaz, 

 seem to be the most promising for a determination of the age of the beds 

 of the» formation. Higher beds, forming a chalky white limestone to the 

 west of Ponce, are also very fossiliferous, but in this area they are sepa- 

 rated from the older rock series by a fault, so that it is quite impossible 

 to tell how far above the base of the formation these beds may lie. It is 

 judged that the portion of the formation seen at Guanica is a still higher 

 horizon, but the exact age values have not been worked out. The total 

 thickness of the whole Tertiary series on the south side of the island is 

 ver}^ great. It was estimated that the shales and marls and limestones 

 in the vicinity of Juana Diaz must certainly amount to three or four 

 thousand feet. For a long distance along the Jacaguas Elver south of 

 Juana Diaz, the beds stand with a dip of approximately thirty-five de- 

 grees toward the south and throughout the greater portion the character 

 is notably different from the beds occurring farther to the west which are 

 judged to overly them. 



To the east of San Juan, along the north coast, there is much less 

 prominence of the Arecibo formation and after passing Eio Piedras it in 

 no place crosses the main road. There are occasional hills somewhat 

 similar to those characteristic of the landscape of Bayamon and vicinity, 

 but they do not reach to so great a height and are separated by very much 

 larger stretches of low ground. The strongest development of this for- 

 mation seen to the east of San Juan is that along the Grande de Loiza 

 Eiver between the railway and the coast. In going still farther east to 

 the vicinity of Luquillo, the inner margin of the formation passes out to 

 sea and the older formation reaches the shore. From this point around 

 the whole eastern end of the island no more outcrops of the x\recibo for- 



